The invention relates generally to the imaging of magnetic fields and more specifically to methods and apparatus for generating images of magnetic fields and for extracting information from images of magnetic fields.
The storage of data in magnetic media such as magnetic tapes, magnetic disks and magnetic stripes has become ubiquitous. One common application involving the storage of digital data in a magnetic medium is the storage of information on magnetic stripe cards. Magnetic stripe cards are commonly used in financial transactions and as a form of identification in conjunction with security systems.
The creation of duplicate magnetic stripe cards is a considerable security concern in magnetic stripe card applications. A duplicate magnetic stripe card that contains the same digital information as an original magnetic stripe card cannot be distinguished from the original on the basis of the digital data stored in the magnetic stripe alone.
U.S. Pat. No. 6,098,881 to DeLand, Jr. et al. describes a system for distinguishing between an original magnetic stripe card and duplicates of the original by using the underlying magnetic characteristics of the materials used in the construction of magnetic stripe cards. The underlying characteristics of the magnetic stripe are virtually impossible to reproduce and measuring these characteristics provides a way of determining whether a magnetic stripe card is an original or a duplicate. U.S. Pat. No. 6,098,881 to DeLand, Jr. et al. is hereby incorporated by reference in its entirety into the present disclosure.
The system described by DeLand, Jr. et al. verifies the characteristics of the magnetic stripe by taking a number of samples using a single fixed magnetic reading head. These samples are then used to generate a one dimensional array of data. The system compares the array of data to a reference array and uses the correlation between the two arrays to determine whether the card being read is an original.
The contents of the array of data for a given magnetic stripe can vary depending on the alignment of the magnetic head that is used to generate the array. Therefore, the correlation between the data array and the reference array required to make a determination that the card being read is an original must be lower in systems that tolerate substantial magnetic reading head misalignment. The lower the required correlation the less ability the authentication system has for distinguishing original magnetic stripe cards from duplicates.